1. Technical Field of the Invention
The present invention relates to acceleration sensors, and particularly, to an acceleration sensor including a piezoelectric material.
2. Description of the Related Art
A known acceleration sensor including piezoelectric ceramics is, for example, disclosed in Japanese Patent No. 2780594, hereinafter referred to as Patent Document 1. Such an acceleration sensor is provided with a bimorph sensor element including a pair of piezoelectric units which are composed of piezoelectric ceramics and are integrally joined to each other in an opposing manner. The sensor element is held inside a casing in a double-supported fashion. When acceleration is applied to the acceleration sensor, the sensor element bends, thus generating stress in the piezoelectric units. The electric charge or voltage generated due to the piezoelectric effect is then detected in order to determine the acceleration. Acceleration sensors of this type are advantageous in view of their compactness and their capability of being formed easily into surface-mounted units (chip units).
In an acceleration sensor based on the above-described principle, a bias current flowing from a circuit is stored in a capacitor C of the piezoelectric material. In order to prevent the circuit from becoming saturated, a resistor R is required for allowing the bias current to be released. However, since the resistor R and the capacitor C define a high pass filter, the acceleration in the frequencies below the cut-off level, such as DC and low frequency, cannot be detected.
On the other hand, an acceleration sensor disclosed in Japanese Unexamined Patent Application Publication No. 2002-107372, hereinafter referred to as Patent Document 2, particularly, the acceleration sensor shown in FIG. 8 in Patent Document 2, includes a single base plate whose opposite sides respectively have first and second resonators formed of a piezoelectric material attached thereto so as to define an acceleration-sensor element, each of the first and second resonators having electrodes on opposite sides thereof. One longitudinal end or both longitudinal ends of the acceleration-sensor element is/are fixed such that the first and second resonators are bendable in their opposing direction in response to acceleration. When the acceleration-sensor element bends in response to acceleration, changes in frequency or changes in impedance in the first and second resonators caused by the bending of the acceleration-sensor element are differentially detected in order to detect the acceleration.
In this case, the acceleration in a DC or low-frequency level can be detected. Moreover, the changes in frequency or the changes in impedance in the two resonators are differentially detected instead of being detected in a separate manner. This counterbalances the stress (for example, a stress caused by a change in temperature) applied to both resonators. Thus, a high-sensitivity acceleration sensor, which is unaffected by, for example, a change in temperature, is achieved. Furthermore, because the central bending plane (i.e. a plane where stress is 0) is set in the base plate, a large degree of tensile stress and compressive stress can be generated in the resonators disposed on the opposite sides of the base plate. Accordingly, this improves the sensitivity of the sensor.
However, because the first and second resonators and the base plate have the same height, namely, the same dimension in a direction perpendicular to the direction in which the acceleration is applied, the first and second resonators have a large cross-sectional area. This means that the tensile stress and the compressive stress generated in the resonators in response to the acceleration cannot be increased. Consequently, this prevents further improvement of the sensitivity (S/N ratio).